Electrical transformer construction



April 1, 1969 KETQ ETAL ELECTRICAL TRANSFORMER CONSTRUCTION Filed Oct.12, 1966 N i J 0 m M m m V00 NH K A .L M 0 hwm i WM A Q WITNESSES 0%.4;? m4.

April 1, 1969 A. l. KETO ET 3ELECTRICAL TRANSFORMER CONSTRUCTION Filedoct. 12. "less United States Patent 3 436 704 ELECTRICAL TRANS FOI QMERCONSTRUCTION August I. Keto, Sharpsville, and Anthony J. Palumbo,

Sharon, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh,Pa., a corporation of Pennsylvania Filed Oct. 12, 1966, Ser. No. 586,116Int. Cl. H01f /14, 27/02 US. Cl. 33670 8 Claims This invention relatesin general to the construction of electrical transformers, and morespecifically to new and improved transformer apparatus of the type whichutilizes at least one partially cast coil, and to new and improvedmethods of constructing the same.

The availability of improved coated metallic foil or strip conductor,and improved casting resins, has made the encapsulation of electricalwindings attractive for electrical transformers. Since foil woundwindings have a single conductor turn per layer, the voltage stressbetween adjacent turns is very low, permitting the use of thin coatingsof electrical insulation, on at least one side of the foil, to insulatethe turn-to-turn voltage stress. Layer insulation, which is insertedbetween the radial layers of turns on wire wound coils, is completelyeliminated. Foil wound windings also have a distinct advantage over wirewound windings from the standpoint of potting or encapsulating thewinding. Since the voltage between layers of Wire wound coils may bevery high, the encapsulating material must impregnate the winding andsupplant all of the air in the voids between the layers.

The viscous nature of encapsulating materials makes it extremelydiflicult to completely impregnate wire wound windings, with the air inthe voids ionizing under high layer-to-layer voltage stresses, producingcorona which degrades the surrounding insulation and generates radiointerference. The low layer-to-layer stresses in foil wound windingsmakes it unnecessary to impregnate the windings, as any air trappedbetween turns will not be subjected to Stresses high enough to causeionization.

Encpsulating foil wound windings in casting resin, however, is notwithout its problems. Care must be taken in the design of the electricalapparatus to provide liberal clearances, and to avoid thin sectioncasting with its inherent air entrapment problems Also, the structure ofthe apparatus, and methods of providing the structure, have all beenvery costly, with high tooling costs accompanying high manufacturingcosts, or the resulting structures are not sound electrically, or acombination of these disadvantages. For example, one method proposed forthe manufacture of electrical transformers is a step-by-step castingprocedure, wherein the inner low voltage coil is wound and cast,degreased and shot blasted, the high voltage coil is wound over the castinner low voltage coil, and this assembly is cast, and the outer lowvoltage coil is wound about the cast high voltage coil. The outer lowvoltage coil is not cast. This method requires expensive, special shapemolds for each casting step, and for each transformer rating, resultingin low production unless a large number of expensive molds are provided.Also, extreme care must be taken to prevent any voids in the high-lowinsulation, which is the cast material disposed between the low voltagecoils and the high voltage coils. Any voids in this area will causecorona and eventual failure of the transformer. Also, with this method,the outer low voltage coil is mechanically protected only by the thincoating of insulation on the foil, with the very real possibility ofdamage to the outer low voltage coil requiring extreme care in thehandling, storage, assembly with the magnetic core, and tanking, whichincreases manufacturing costs and rejects.

Another method proposed eliminates the step-by-step casting procedure bydisposing spacers between the windings and then encapsulating. Testresults, however, have shown excessive corona and inability to withstandtest voltages. Manufacturing tolerances on the coils cause voids betweenthe spacers and the coils which are not filled with the potting resin.These voids produce the corona. This type of winding is also slow andcostly to manufacture, due to the extreme care required in spacing thecoils, and the encapsulating step required to produce a structure whichis corona free.

Therefore, it would be desirable to provide new and improved transformerapparatus, and methods of assembly, which eliminate the necessity ofhaving a large number of special shape molds in order to obtain highproduction, which provides void-free high-low insulation between the lowand high voltage coils, which relaxes the dimensional tolerances on thewindings without producing voids in which corona may form, whichstrengthens the winding structure without using an excessive amount ofcasting material, and which protects the outside low voltage windingagainst mechanical damage.

Accordingly, it is an object of the invention to provide a new andimproved transformer structure which utilizes at least one partiallycast coil or winding.

Another object of the invention is to provide a new and improvedtransformer structure which utilizes cast resin to encapsulate at leasta portion of one of its windings, and to strengthen the structureagainst short circuit stresses, without encapsulating the whole windingstructure.

A further object of the invention is to provide a new and improvedtransformer structure which utilizes new and improved solid insulatingmeans between the high and low voltage windings, with the thickness ofthe insulating means providing the necessary high-low insulation withoutregard to and without detriment from any air spaces between the solidinsulating means and adjacent winding structures.

Another object of the invention is to provide a new and improved methodfor constructing electrical transformers in which one of its coils is atleast partially encapsulated with cast solid insulating means.

Still another object of the invention is to provide a new and improvedmethod for constructing transformers in which at least a portion of theinsulation on at least one of its coils is provided by cast solidinsulation, and which does not require the use of specially shapedmolds.

Briefly, the above cited objects are accomplished by providingpre-formed high-low insulation members. Electrically conductive coatingson opposite sides of the highlow insulating members are electricallyconnected to the immediately adjacent winding, providing a substantiallyzero potential gradient between the windings and the insulating member.The ends of the high voltage winding are cast in solid insulation whichcompletes the insulation for the high voltage winding and strengthensthe structure against short circuit stresses. Circumferentially spacedapart ends of the high-low insulating structures provide ducts throughthe winding structure which allow cast solid insulating means to beintroduced at one end of the structure, and use the ducts to flow to theopposite end, thus encapsulating both ends of the high voltage windingin one casting operation. The ducts are also filled with the cast solidmaterial, which precludes the generation of corona therein.

The method of manufacture, in one of the embodiments of the invention,eliminates costly specially shaped molds for casting the ends of thehigh voltage windings in solid insulation. In this embodiment of theinvention the ends of the low voltage windings project past the highvoltage winding at both ends of the structure, which automaticallyprovide cavities or molds for the cast solid insulation, except formeans for sealing the bottom portion of the assembly until the resinousmaterial is at least precured.

Further objects and advantages of the invention will become apparentfrom the following detailed description, taken in connection with theaccompanying drawings, in which:

FIGURE 1 is an elevational view, partially in section, of the inner lowvoltage winding, whose construction is the first step in the methodtaught by the invention,

FIG. 2 is an elevational view, partially in section, which illustratesthe placement of the first pre-formed high-low insulating structurerelative to the low voltage winding assembly shown in FIG. 1,

FIG. 3 is an elevational view, partially in section which illustratesthe position of the high voltage winding assembly relative to theassembly shown in FIG. 2,

FIG. 4 is an elevational view, partially in section. which illustratesthe placement of the second pro-formed highiow insulating structurerelative to the assembly shown in FIG. 3,

FIG. 5 is an elevational View, partially in section, which illustratesthe disposition of the outer low voltage winding relative to theassembly shown in FIG. 4,

FIG. 6 is a cross-sectional view of the assembly shown in FIG. 5, takenalong the line V1VI,

FIG. 7 is an elevational view, partially in section, which illustratesthe assembly shown in FIG. 5 after casting,

FIG. 8 is an elevational view, partially in section, which illustratesthe assembly shown in FIG. 7 with an outer protective coating ofelectrical insulating material,

FIG. 9 is a pictorial view which illustrates more clearly the twopre-formed high-low insulating structures and their relationshiprelative to one another, and

FIG. 10 is a schematic diagram which illustrates the electricalconnections from the high and low voltage windings to the electricallyconductive coatings on the preformed high-low insulating structures.

Referring now to the drawings, and FIG. 1 in particular, there is shownan elevational view, partially in section, of a coil or winding 10 whichmay be used as the inner low voltage winding of a transformerconstructed according to the teachings of the invention. Winding 10 isformed of an electrically conductive metallic strip, sheet or foilconductor having a predetermined width dimension, such as copper oraluminum, and having a predetermined number of conductor turns 12, shownin the magnified insert 11, which are continuously wound upon a mandrel,or insulating tube and mandrel, to provide a predetermined inner opening14 sized to receive the leg portion of magnetic core means (not shown).The edges of the strip material form first and second coaxially disposedend surfaces 13 and 15, on the substantially tubular winding structure10. As is the characteristic of foil wound coils, each layer of the coilincludes but one turn, with the turns being separated from one anotherby insulatin g means 16, shown in the magnified insert 11. Insulatingmeans 16, shown in the magnified insert 11. Ining enamel, disposed onone or both of its major sides, such as an epoxy base enamel, or it maybe a thin film of electrical insulation, such as Mylar, which is woundwith the foil conductor to interleave the conductor turns 12. Electricalinsulating means 16 may be extremely thin, which contributes to a highspace factor for foil wound coils, as the layer-to-layer voltages andthe turn-to-turn voltages are one and the same, and are very lowrelative to the high layer-to-layer voltage stresses found in wire woundcoils.

After the step of winding the inner low voltage coil 10, a pre-formedinsulating structure 20, having inner and outer major surfaces 21 and23, respectively, is disposed about the inner low voltage winding 10, asshown in FIG. 2. Insulating structure forms the first highlow insulatingmeans, and may be formed in any suitable manner. For example, preformedinsulating structure 20 may be formed of a butyl rubber, or a resin,such as an epoxy, and it may be flexible or a cast solid.

If flexible, it is simply wrapped about the inner low voltage coil 10.If a cast solid, it is telescoped over the inner low voltage winding orcoil 10. The pre-for-rned solid insulating structure 20 has manyadvantages. For example, when forming the high-low insulation by castingin place between the high and low voltage windings, there are manyfactors which cannot be accurately controlled which contribute tonon-uniformity in the cast structure from unit to unit, with air voidsbeing formed in some or all of the units which lowers the coronainception voltage. By using pre-formed high-low insulation, itsmanufacture may be carefully and closely controlled to provide avoid-free structure specifically formulated for the particular high-lowvoltage stress to be encountered, without compromising its electricalcharacteristics in order to obtain other characteristics necessary whencasting the high-low insulation between the high and low voltagewindings.

The pre-formed high-low insulation structure 20 also has many otheradvantages, both from cost and functional viewpoints.

If pre-formed insulation structure 20 is flexible, its length ispredetermined to provide a gap or opening 22 between its ends when woundabout low voltage winding 10. Or, if insulating structure 20 is cast,the wall section is not continuous, with the insulating structure 20having first and second circumferentially spaced ends which provide agap 22. The ends may be tong-ued or ribbed, as shown more clearly inFIGS. 6 and 9, for purposes which will be hereinafter explained.

Insulating structure 20 also has electrically conductive coatings 26 and28 disposed on the inner and outer major surfaces 21 and 23,respectively. Coatings 21 and 23 are electrically isolated from oneanother, and may be disposed such that they remain a predetermineddistance from the edges of the structure, to increase the creep distancebetween the coatings. Thus, the first function of the gap or opening 22in the wall section of insulating structure 20 is to prevent theelectrically conductive coatings 26 and 28 from forming a short circuitabout the low voltage winding 10 and winding leg portion of the magneticcore. The second function of the opening 22, which will be fullydescribed hereinafter, is to provide a duct for liquid casting resin inthe casting step of the assembly of the apparatus.

The electrically conductive coatings 26 and 28 may be in the form ofexcellent electrical conductors, such as copper or aluminum, or they maybe semiconductive, i.e., having a voltage dependent resistivity, such ascoatings containing particulated silicon carbide, or they may bepartially conductive, such as coatings containing particulated carbon.

The inner electrically conductive coating 26 is electrically connectedto the outer conductive turn of low voltage coil 10 by electricalconnection 29. Therefore, the electrically conductive coating 26 is atsubstantially the same potential as the outer conductor turn of winding10. Thus, it is not essential that the insulating structure 20 andcoating 26 be disposed tightly against winding structure 10. Any air orvoid between coating 26 and winding 20 will be at substantially zeroelectrical stress or potential gradient. The high-low insulatingstructure 20 may thus be cast and the winding structure 10 wound torelaxed tolerances, which lowers manufacturing costs, as any spacebetween the insulating structure and winding will not be subjected to acorona inducing stress. As will hereinafter be explained, all of thehigh-low stress will be applied to the high-low insulating structure 20.Coating 26 also provides a smooth, rounded, equipotential surface whichprevents the concentration of voltage gradients about the edges of thewinding and other sharp surfaces.

The next step in the method of forming a transformer according to theteachings of the invention is shown in FIG. 3. FIG. 3 illustrates theplacement of the high voltage winding structure 30 relative to lowvoltage Winding 10 and the high-low insulating structure 20. Highvoltage winding structure 30 has a predetermined number of conductorturns and has a predetermined longitudinal dimension in the direction ofthe width of the electrically conductive strip, which in this embodimentof the invention is preferably less than the width dimension of thestrip of which. the low voltage Winding is wound.

High voltage winding 30 may be formed by continuously winding anelectrically conductive strip or foil upon the high-low insulatingstructure 20, until providing the required number of conductor turns.High voltage winding 30 may be formed of electrically conductive stripand the turns insulated, similar to the low voltage winding 10,hereinbefore described, with the edges of the strip forming first andsecond coaxially disposed spaced end surfaces 35 and 37.

High voltage winding 30 has its inner conductor turn electricallyconnected to the outer electrically conductive coating 28 via conductor32, which reduces the electrical stress between the inner turn ofwinding 30 and coating 28 to substantially zero, and applies the totalelectrical stress between windings 20 and 30' to the insulatingstructure 20.

In some instances, high voltage winding 30 may be formed of two or moreaxially spaced, electrically connected sections, with each section beingformed of electrically conductive strip. In this event, coating 28 willalso be formed to have the same number of axially spaced sections as thehigh voltage winding, with the coatings being isolated electircally fromone another, and which are electrically connected to the outer turn ofits adjacent high voltage section. In this event, the axially spacedhigh voltage winding section, in this embodiment of the invention,:would still have a combined width which is less than the width of thelow voltage winding 10.

The next step in the transformer assembly is shown in FIG. 4, andcomprises the placement of a second highlow insulating structure 40about high voltage winding 30. Insulating structure 40 is similar inconstruction to the high-low insulating structure 20, hereinbeforedescribed, except for having an inner opening sized to encompass theouter dimension of high voltage winding 30 or, if flexible, a lengthcapable of encircling high voltage winding 30, except for a gap oropening 42 similar to the gap 22 provided in insulating structure 20.Insulating structure 40 has inner and outer major surfaces 44 and 46,respectively, and similar to insulating structure 20, has anelectrically conductive coating 47 disposed on its inner surface 44, andan electrically conductive coating 49 disposed on its outer surface 46.Electrically conductive coatings 47 and 49 may be formed as hereinbeforedescribed relative to coatings 26 and 28. Insulating structure 40 may bewrapped about high voltage winding 30, if flexible, or teleseopedthereover, if it is a cast solid. The inner electrically conductivecoating 49 is electrically connected to the outer turn of high voltagewinding 30, via electrical conductor 48, which reduces to substantiallyzero the potential gradient between coating 47 and winding 30, applyingthe electrical stress to the high-low insulating structure 40, andproviding a smooth, equipotential surface which aids in reducingconcentrations of electrical stress.

The next step in the assembly and construction of a transformeraccording to the teachings of the invention is shown in FIG. 5. Thisstep involves the coaxial placement of the outer low voltage winding orcoil 50 relative to the assembly shown in FIG. 4, which thus forms thecomplete winding assembly 60. The outer low voltage winding 50 may becontinuously wound about the highlow insulating structure 40, and isformed of conductive strip or foil, similar to the inner low voltagewinding 10, having a predetermined width and a predetermined number ofcontinuous conductor turns separated by suitable insulating means. Theouter low voltage winding 50, like the inner low voltage winding 10,preferably extends past the high voltage winding 30 at both endsthereof, for a predetermined distance. The edges of the conductive stripform first and second coaxially disposed, spaced end surfaces 55 and 57.

In order to form an area of substantially zero potential gradientbetween the outer low voltage winding 50 and insulating structure 40,and confine the electrical stress between windings 50' and 30 toinsulating structure 40, which is specially prepared for this function,the inner turn of outer low voltage winding 50 is electrically connectedto the outer conductive coating 47 on insulating structure 40 viaelectrical conductor 52.

Winding structure 60 is now physically ready for the casting operation.However, since foil or strip wound coils undergo considerable growthduring the casting and curing cycles, it is preferable to include meansfor consolidating the windings and stabilizing their dimensions beforecasting. This consolidating and stabilizing step may be accomplshed byincluding a thin coating of a suitable adhesive disposed on at least oneof the major surfaces of the conductive foil of which the windings 10,30 and 50 are wound. This consolidation and stabilizing of foil woundcoils is described in detail in co-pending application Serial No.506,350, filed Nov. 4, 1965, now abandoned, which is assigned to thesame assignee as the present application. If the coils include suchconsolidating means, such as an epoxy adhesive, the next step in themethod of constructing the transformer would be to heat the windingassembly 60 to fiow, set, and cure the adhesrve.

FIG. 6 is a cross-sectional view of the winding assembly 60 shown inFIG. 5, taken along the line VIVI, which more clearly illustrates theducts formed in winding structures 60 by openings 22 and 44 provided bythe circumferentially spaced ends of insulating structures 20 and 40,respectively. These ducts or openings 22 and 42 are used to provide castsolid insulation at both ends of the winding structure '60 with a singlestep casting operation. In other words, by orienting winding structure60 as shown in FIG. 5, the liquid casting resin may be introduced at thetop of the structure, in the natural depression or cavity formed by theportions of the low voltage windings 10 and 50 which extend past thehigh voltage winding 30, and this casting resin will flow downwardlythrough openings 22 and 42 to fill the cavities formed at the bottom ofthe winding structure 60 by the portions of low voltage windings 10 and50 which extend past high voltage winding 30. Thus, in the casting step,costly, specially shaped molds are not required, as in this embodimentof the invention the construction of winding assembly '60 automaticallyforms the major portion of the mold for containing cast resinousinsulation, which will insulate the ends of the high voltage winding 30from ground and from the inner and outer low voltage windings .10 and30. It is only necessary to place the winding structure in a very simplemold which will seal the bottom of winding assembly 60 during thecasting step. If the windings are not formed to automatically providecavities for containing the cast solid insulation to insulate the endsof high voltage winding 30, winding assembly 60 may be placed in a moldwhich will provide sufiicient space at each end of the winding assemblyfor the cast solid insulation.

After placing winding structure 60 into a suitable mold, the liquid castresinous material is introduced into the top of the mold, the castresinous insulation is directed to the bottom of the mold throughopenings or ducts 22 and 42, to fill the space about the lower end ofhigh voltage winding 30. In order to provide ample duct space without anexcessive gap between the spaced ends of the insulating structures, theends may be tongued, as il lustrated in FIG. 6. In other words, the wallthickness of insulating members 20 and 40 may be reduced for apredetermined distance adjacent their ends, to increase thecross-sectional area of the ducts and facilitate resin flow through theducts. After filling the bottom space in the mold, openings 22 and 42fill with the cast resin, and then the space about the upper end of highvoltage winding 30 is filled with the cast resin. Winding assembly 60',after the cast resin is introduced, is shown in the elevational view,partially in section, of FIG. 7. FIG. 7 illus trates how the cast resincompletes the insulating requirements of the high voltage windings 30 atthe bottom of the structure 60 by cast solid insulating means 62, and atthe top of structure 60 by cast solid insulating means 64. The castsolid insulation means 62 and 64 adheres to the pre-formed insulatingstructures 20 and 40, to high voltage coil structure 30, and to the endsof low voltage windings and 50, to form a sealed, solid insulatingsystem in which the solid insulation is stressed in puncture, and not increep.

After the cast insulating means is introduced into the winding structure60, the cast resin is cured by a suitable heating cycle. For example, asuitable heating cycle using an epoxy resin system, would be 2-4 hoursat a predetermined temperature to set the resin system, after which themold may be removed and reused, and 8 hours at a predeterminedtemperature to cure the resin system.

Many resin systems may be used for forming the cast solid insulatingmeans 62 and 64, which should preferably be thermosetting, althoughthermoplastic resin systems may also be used if their softeningtemperature is above the operating temperature range of the completedtransformer.

In general, the resin system should be rigid, have a low coeflicient ofthermal expansion tailored to closely match that of the conductive foilof which the high voltage winding 30 is formed, have excellent crackresistant characteristics, and a high thermal conductivity. An epoxysystem found to be excellent is described in detail in co-pendingapplication Serial No. 456,038, filed May 6, 1965, which is assigned tothe same assignee as the present application.

Winding assembly 60 is now complete, except for mechanically protectingthe exposed surfaces of low voltage windings 10 and 50 during thevarious steps in the assembly and handling of the transformer. Thismechanical protection is provided, as shown in FIG. 8, by applying athin coating of electrical insulation 70 about the whole windingassembly 60. Coating 70 may be applied by spraying, fluidized bedcoating, or in any other suitable manner, and may be any suitableelectrical insulating material, such as an epoxy system, or systemcontaining butadiene-styrene, or silicone rubber.

Winding assembly 60 is thus completed, and as shown in FIG. 8, may beassembled with a suitable magnetic core assembly 80, which is shown indotted outline, to form a core-winding assembly 90 which may be disposedin a suitable tank filled to a predetermined level [with a fluidinsulating and cooling dielectric (not shown), such as oil.

Thus, a transformer 100 is formed, which utilizes cast insulation in away which derives all of its benefits, without its accompanyingdrawbacks. This beneficial arrangement is obtained by using twopre-formed insulating tructures 20 and 40, shown pictorially in FIG. 9to more clearly illustrate their adjacent, spaced, concentric placement,and openings 22 and 42 which interrupt the conductive coatings 26, 28,47 and 49, preventing a short circuit about the windings and core, andwhich also serves to enable the casting operation to be formed in onestep. FIG. 9 also illustrates the construction of insulating structures20 and 40* more clearly, illustrating the inner and outer major sides 21and 23, respectively, of insulting structure 20 and how the structureforms the opening or gap 22 by first and second circumferentially spacedends 91 and 92, respectively, which are disposed in spaced relation.This view also clearly illustrates the inner and outer major surfaces 44and 46, respectively, of insulating structure 40 and how opening 42 isformed by first and second circumferentially spaced ends 93 and 94-. The

pre-formed insulating structures 20 and 40 may be specially prepared to:withstand the specific high voltage stresses encountered in a specificapplication, and it may be prepared without the deleterious voids whichmay occur when casting thin high-low insulation sections in place. Thus,thin section casting, with its inherent thin section air entrapmentproblems, is completely eliminated, the pre-formed insulating structurewill perform the required function better, and the thickness of theinsulating structures 20 and 40 may be selected for the particularelectrical stresses of the specific transformer, without resorting tocostly tooling and mold changes. Thus, changeover from one type oftransformer rating to another may be quickly and efficiently made.

The use of pre-formed insulating structures, along with their conductivecoatings, enables all of the electrical stress between the windings tobe applied to the insulating structures 20 and 40, and makes itunnecessary to impregnate any space between the pre-formed insulatingstructures and the adjacet windings. Since the turns of each of the foilwindings adjacent to a conductive coating is electrically connectedthereto, which is schematically illustrated in FIG. 10, any spacebetween the windings and electrically conductive coatings hassubstatially zero potential gradient, which precludes the formation ofcorona. FIG. 10 also illustrates terminals 99 and 101 on low voltagewinding 10, terminals 97 and 98 on high voltage winding 30, andterminals and 96 on low voltage winding 50, which have been omitted onthe other views of these windings for purposes of simplicity.

The smooth curved electrically conductive coatings form equipotentialsurfaces which prevent concentrations of electrical stress and shieldthe sharp edges of high voltage winding 30.

Further, the disclosed construction lends itself to low cost progressivewinding of the transformer, whereby each transformer winding is woundabout the prior assembled section, and the pre-formed insulatingstructures, with the discotninuous wall sections, allow the casting tobe performed in one step. The disclosed construction requires a minimumin the way of special tooling and molds, enabling quick changes to bemade in the dimesions and ratings of transformers with minimum cost andchangeover time.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it is intended that all matter contained in theforegoing description or shown in the accompanying drawings, shall beinterpreted as illustrative and not in a limiting sense.

We claim as our invention: 1. Electrical inductive apparatus comprising:first, second and third winding structures disposed in spaced, adjacent,concentric relation, respectively;

said first, second and third winding structures each having apredetermined number of conductor turns formed of electricallyconductive strip material having predetermined width dimensions;

said first, second, and third winding structures each having first andsecond coaxially disposed end surfaces formed by the edges of saidelectrically conductive strip material;

first and second solid insulating means disposed between said first andsecond winding structures, and between said space second and thirdwinding structures, respectively; said first and second solid insulatingmeans having first and second coaxially disposed end portions, first andsecond circumferentially spaced end portions, which form ducts extendingbetween said first and second coaxially spaced end portions, and innerand outer major surfaces which have an electrically conductive meansdisposed thereon; means electrically connecting the electricallyconductive means on said inner and outer major surfaces to the windingstructure immediately adjacent thereto; and

cast solid insulating means disposed about the first and second endsurfaces of at least said second winding structure, and in the ductsformed by the circumferentially spaced end portions of said first andsecond solid insulating means.

2. The electrical inductive apparatus of claim 1, including means formechanically protecting the exposed surfaces of said first and thirdwinding structures, said means being in the form of a coating of solidelectrical insulation.

3. The electrical inductive apparatus of claim 1, wherein the widthdimensions of the electrically conductive strip of which said first andthird winding structures are formed, exceed the width dimension of theelectrically conductive strip material of which said second windingstructure is formed, providing a winding assembly having a cavity ateach end thereof defined by the first and second edge surfaces of saidsecond winding structure and the portions of said first and thirdwinding structures which extend past the first and second edge surfacesof said second winding structures, said cavities containing said castsolid insulating means.

4. The electrical inductive apparatus of claim 3, including electricalinsulating coating means disposed over the outer surfaces of the windingassembly formed by said first, second and third Winding structures.

5. A method of constructing an electrical winding assembly, comprisingthe step of: Winding a strip of electrically conductive material havinga predetermined width dimension, to form a first low voltage winding;providing a first pre-formed solid insulation structure having first andsecond major surfaces, each having an electrically conductive coatingthereon, and first and second end portions; disposing said firstpre-formed solid insulating structure around said first low-voltagewinding, with its first and second end portions being circumferentiallyspaced apart; electrically connecting the electrically conductivecoating disposed adjacent to said first low voltage Winding, to saidfirst low voltage winding; winding a strip of electrically conductivematerial having a predetermined width dimension on said first pre-formedsolid insulating structure to form a high voltage winding having firstand second coaxially spaced end surfaces, and also complete a first ductbetween the first and second circumferentially spaced ends of said firstpre-formed solid insulating structure; electrically connecting theremaining electrically conductive coating on said first pre-formed solidinsulating structure to said high voltage winding; providing a secondpreformed solid insulating structure having first and second majorsurfaces, each having an electrically conductive coating thereon, andfirst and second end portions; disposing said second pre-formed solidinsulation structure around said high voltage winding, with its firstand second ends being circumferentially spaced apart; electricallyconnecting the electrically conductive coating disposed adjacent to saidhigh voltage winding, to said high voltage winding; winding a strip ofelectrically conductive material having a predetermined width on saidsecond preformed solid insulating structure, to form a second lowvoltage winding and also complete a second duct between the first andsecond circumferentially spaced ends of said second pre-formed solidinsulating structure; electrically connecting the remaining electricallyconductive coating on said second pre-formed solid insulating structureto said second low voltage Winding; casting a resinous insulatingmaterial about the first and second coaxially disposed ends of said highvoltage winding, by introducing said resinous material at one end ofsaid high voltage winding and using said first and second ducts todirect said resinous material to the other end; and curing said resinousmaterial to form cast solid insulation which insulates the ends of saidhigh voltage winding from said low voltage windings and ground, andwhich fills said first and second ducts to prevent the formation ofcorona therein.

6. The method of constructing the electrical winding assembly of claim5, including the step of coating the exposed portions of said first andsecond low voltage windings with electrical insulating means, formechanical protection.

7. The method of constructing the electrical winding assembly of cliam-5 wherein the strip width dimension of which said first and second lowvoltage windings are wound exceeds the strip width dimension of whichsaid high voltage winding is wound, which provides a cavity at each endof said high voltage winding for receiving said cast resinousinsulation.

8. The method of constructing the electrical winding assembly of claim 5wherein the strip of which said first and second low voltage, and saidhigh voltage windings are formed has a coating of adhesive on at leastone side thereof, and including the step of heating said first andsecond low voltage and high voltage windings prior to the step ofcasting the resinous insulating material, to consolidate said windingsand stabilize their dimensions.

References Cited UNITED STATES PATENTS 1,837,245 12/1931 Wheeler 336-XR2,998,583 8/ 1961 Worcester 336-206 XR 3,084,299 4/1963 Lord 336-703,265,998 8/1966 Park 33670 LEWIS H. MYERS, Primary Examiner. THOMAS J.KOZMA, Assistant Examiner.

U.S. Cl. X.R. 29-605; 33684, 96, 232

1. ELECTRICAL INDUCTIVE APPARATUS COMPRISING: FIRST, AND SECOND ANDTHIRD WINDING STRUCTURES DISPOSED IN SPACED, ADJACENT, CONCENTRICRELATION, RESPECTIVELY; SAID FIRST, AND SECOND AND THIRD WINDINGSTRUCTURES EACH HAVING A PREDETERMINED NUMBER OF CONDUCTOR TURNS FORMEDOF ELECTRICALLY CONDUCTIVE STRIP MATERIAL HAVING PREDETERMINED WIDTHDIMENSIONS; SAID FIRST, SECOND, AND THIRD WINDING STRUCTURES EACH HAVINGFIRST AND SECOND COAXIALLY DISPOSED END SURFACES FORMED BY THE EDGES OFSAID ELECTRICALLY CONDUCTIVE STRIP MATERIAL; FIRST AND SECOND SOLIDINSULATING MEANS DISPOSED BETWEEN SAID FIRST AND SECOND WINDINGSSTRUCTURES, AND BETWEEN SAID SPACE SECOND AND THIRD WINDING STRUCTURES,RESEPECTIVELY; SAID FIRST AND SECOND SOLID INSULATING MEANS HAVING FIRSTAND SECOND COAXIALLY DISPOSED END PORTIONS, FIRST AND SECONDCIRCUMFERENTIALLY SPACED END PORTIONS, WHICH FORM DUCTS EXTENDINGBETWEEN SAID FIRST AND SECOND COAXIALLY SPACED END PORTIONS, AND INNERAND OUTER MAJOR SURFACES WHICH HAVE AN ELECTRICALLY CONDUCTIVE MEANSDISPOSED THEREON; MEANS ELECTRICALLY CONNECTING THE ELECTRICALLYCONDUCTIVE MEANS ON SAID INNER AND OUTER MAJOR SURFACES TO THE WINDINGSTRUCTURE IMMEDIATELY ADJACENT THERETO; AND CAST SOLID INSULATING MEANSDISPOSED ABOUT THE FIRST AND SECOND END SURFACES OF AT LEAST SAID SECONDWINDING STRUCTURE, AND IN THE DUCTS FORMED BY THE CIRCUMFERENTIALLYSPACED END PORTIONS OF SAID FIRST AND SECOND SOLID INSULATING MEANS.